This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The cytoplasm of a cell mediates important cellular functions such as intracellular transport, cell communication and cell motility. We are interested in how the organization of the cytoplasm affects or controls these functions. Specifically, it has been shown that the cytoplasm of fission yeast cells undergoes a kind of "phase transition" depending on external control parameters. Yeast cells enter a solid-like state when grown in nutrient deficient conditions. We are interested in the mechanism that leads to this "freezing" of cells. The only apparent way that the cell can cause the immobilization of all structure is to form some kind of rigid polymer network. A family of proteins (septins) has been identified as filament forming and is the most likely candidate to cause the transition from the fluid- to the solid-like state. Preliminary data leads to the hypothesis that the septin protein Spn1 forms a rigid network in cells grown in starvation conditions. We will use electron microscopy of such cells to visualize, for the first time, a septin protein network.